40 



INBEEEDING AND OUTBREEDING 



Reduction of the chromosomes takes place in plants 

 just as it does in animals, but the introduction of a gamete 

 generation, the gametophyte, complicates matters. In 

 the seed plants, pollen mother cells are produced in the 

 anthers of the flower which go through precisely the 

 same divisions as in animal spermatogenesis (Fig. 11). 

 But each of the four nuclei thus produced divides during 



Fig. 11. — Formation of pollen grains in the lily. B, stages in the formation of pollen 

 grains in a group of four (tetrad) within the pollen mother cell; C, mature pollen grain 

 with early stages in the development of the male gametophyte; t, tube nucleus; g, generative 

 nucleus. (After Bergen and Davis.) 



the formation of the pollen grain, forming a generative 

 and a tube nucleus. The tube nucleus it is that germinates 

 and passes down the style when the pollen grain falls on 

 a ripe stigma. During this period of pollen tube growth 

 the generative nucleus passes down through it toward the 

 ovule, and while so doing divides again, leaving two nuclei 

 each wdth a function to perform. One fuses with the egg 

 and the other with the so-called endosperm nucleus, com- 



